With a career spanning almost 50 years, Peter Kennard is without doubt Britain’s most important political artist and its leading practitioner of photomontage. His adoption of the medium in the late 1960s restored an association with radical politics, and drew inspiration from the anti-Nazi montages of John Heartfield in the 1930s. Many of Kennard’s images are now themselves icons of the medium, defining the tenor of protest in recent times and informing the visual culture of conflict and crisis in modern history.

Kennard defines his role as that of a “communicator” and is determined to make art that exists outside the confines of the art world, once stating that: “For me, getting the work out into the world and used is as important as its production.”

This has served as both maxim and method for Kennard and since the early 1970s he has brought his art to street level, either as fly posters, protest placards or T-shirts in support of a variety of groups, including the CND and Amnesty International.

Born in London in 1949, Kennard painted from the age of 13, using a coal shed as a makeshift studio. After securing a scholarship to attend Byam Shaw art school in London he undertook further study at the Slade and at the Royal College of Art.

It was at the Slade that Kennard underwent his political awakening. It was 1968, a year of youthful insurrection against the status quo. The art Kennard produced formed the basis of his career, as he recounted later: “I studied as a painter, but after the events of 1968 I began to look for a form of expression that could bring art and politics together to a wider audience … I found that photography wasn’t as burdened with similar art historical associations.”

The result was his STOP montage series. The 31 works combined numerous, often classic, photographs of contemporary events with a myriad acetate overlays of abstract marks. Kennard sought in part to capture the disorientating atmosphere of the era as he experienced it himself as a student activist. The series also reflected his interest in Bertolt Brecht’s theory of a Verfremdungseffekt (“distancing effect”), whereby events are stripped of familiar attributes to create fresh curiosity and astonishment.

Visual shorthand for Blair’s controversial Iraq policy … Photo Op, made by Cat Phillipps as part of the collaborative practice kennardphillipps. Photograph: kennardphillipps

These experimental pieces, with their exploration of distorted perceptions and perspectives, were typical of his dynamic milieu in the late 1960s. In the 70s, however, Kennard’s simpler, starker imagery sought to rase awareness of human rights violations in Chile and Northern Ireland. These montages found a regular platform in the leftwing daily, Workers’ Press, until disillusionment and editorial interference put paid to his involvement.

His work attained an early maturity in the 80s, amid the rising tensions of the cold war and the divisive policies of Margaret Thatcher. Direct and often sardonic montages were made for CND, articulating fears in British society as the east-west stand-off pushed the world towards potential nuclear catastrophe. This culminated in Kennard’s transposition of John Constable’s Haywain, shown bristling with American cruise missiles in response to their deployment in Britain. On the strength of this imagery, Ken Livingstone, then leader of the Greater London Council, commissioned Kennard to give visual expression to his declaration that the capital was a “nuclear-free zone”. Target London, a folio of 18 posters, bleakly satirised the Thatcher government’s Protect and Survive nuclear attack directives; the critic Richard Cork described the series as the “most hard-hitting attack on government imbecility”.

In 1989 the fall of the Berlin Wall gave Kennard, like many, cause for hope. However, the emergence of the so-called “new world order” quickly dampened this initial optimism. He began experimenting with the creation of a number of three-dimensional artworks, later explaining that these arose from “a mixture of personal experience, disillusion with organised politics and the use by the media of innumerable digital photomontages” causing him to “question the effectiveness of photomontage as a critical, social probe”.

Works such as Welcome to Britain, an installation of placards and crates at the Royal Festival Hall, and Reading Room, an arrangement of newspaper lecterns shown originally at Gimpel Fils gallery in London, contemplated aspects of the developing post-cold war, pre-millennium society, from Britain’s dispossessed and homeless to the supremacy of the stock markets.

The Iraq war in 2003 prompted Kennard to reconnect with photomontage. A collaboration with Cat Phillipps used digital technology to create one of the archetypal images of the conflict. Photo Op, picturing a grinning Tony Blair posing for a selfie in front of burning oil wells in an arid landscape, became a visual shorthand for Blair’s controversial Iraq policy. Before this montage, Kennard created his Decoration paintings, a series of 18 three-metre high canvases that drew attention to the human cost of the war while simultaneously meditating on tokens of commemoration and military valour. Generated by a combination of digital printing and oil paint, the Decoration series’ concern with surface and finish also signalled Kennard’s desire to connect with painting. This too was emphasised by his series, Face, a group of 28 anonymous portraits which, merging in and out of darkness, stood for the voiceless and marginalised in a fragmented world.

Both series reveal the contemplative nature of Kennard’s mature work. Now entering his later career, the artist has had cause for reflection on an oeuvre dedicated to the political and social causes of a turbulent half-century. This has inspired his latest work, Boardroom, an installation dwelling on aspects of modern conflict that incorporates some of his most familiar images and motifs.

Boardroom will debut at the Imperial War Museum London as part of its retrospective exhibition Peter Kennard: Unofficial War Artist. This is the latest chapter in a long association between artist and museum; since the 1980s, the museum has collected Kennard’s work, recognising his ability to tap into the zeitgeist and resonate with the British public.

In 1990 it hosted a major exhibition of his work, Images for the End of the Century. The show reflected the first 20 years of the artist’s career, including work that contrasted the nuclear arms race with the plight of developing nations and the west’s continued support of brutal dictatorships. The new exhibition will address the subsequent 20 years, incorporating Kennard’s socially aware art forms, developed through his exploration of three-dimensional media and installations. It will capture, too, his recent reconnection with the painted surface, revisiting hitherto unexhibited student paintings that reveal his fascination with Bacon, Goya, Giacometti and Rembrandt in an extended celebration of the output and outlook of this unique, provocative and restlessly inventive British artist.

• Peter Kennard: Unofficial War Artist is at the Imperial War Museum, London SE1, from 14 May. iwm.org.uk. An accompanying book will be published on 31 May and is available from www.iwmshop.org.uk

• This article was amended on 4 May 2015. An earlier version carried a headline that referred to “the man who made Blair’s selfie” and the main photograph was a photomontage of Tony Blair taking a selfie in front of a burning oil field. The photomontage, which is not part of the Imperial War Museum’s exhibition, was made by Cat Phillipps in 2005 as part of the collaborative practice kennardphillipps.